This invention relates generally to the fabrication of microelectronic devices, and more specifically to methods for forming etch stop and barrier layers with low dielectric constants and to the composition of the etch stop layers.
Microelectronic integrated circuits based on patterned semiconductor materials are continuing to evolve towards devices with an extremely high density of circuit elements per unit volume. As the features of these devices are reduced to smaller sizes, the performance of the materials that constitute the device will critically determine their success. One specific area in need of advancement is the electrical insulator used between the wires, metal lines, and other elements of the circuit. As the distances between the circuit elements become smaller, there will be increased problems due to capacitive coupling (crosstalk) and propagation delay. These difficulties can be avoided by preparing the circuit using an insulating material that possesses a dielectric constant as low as possible. In particular, it is desired that the insulating material possess a dielectric constant below the dielectric constant of about 4 of silicon dioxide, the material that has long been used in integrated circuits as the primary insulating material.
In addition to using low dielectric constant (low k) insulating materials, the next generation of devices will increasingly use copper for the metal interconnects because of the lower resistivity of copper as compared with aluminum, the conductor in current use. In order to prevent diffusion of copper between metal levels in multilevel devices and to prevent diffusion of copper into adjacent dielectric layers, a barrier layer is provided below each copper interconnect level. The barrier level also serves as an etch stop layer in the dielectric etch of the vias, which is a necessary step in the process of fabricating devices. Currently, silicon nitride is typically used as the material for the barrier layer. However, silicon nitride has a dielectric constant around 7. In order to fully exploit low k insulating materials and copper interconnects, an etch stop barrier material with a dielectric constant lower than that of silicon nitride is desired.
Furunura et al. in U.S. Pat. No. 5,103,285 and Loboda et al. in U.S. Pat. No. 5,818,071 teach the use of silicon carbide as an alternative diffusion barrier layer. However, the silicon carbide film conventionally formed by chemical vapor deposition of silane and methane, which typically contains hydrogen, is characterized by high leakage current and low breakdown field. Values for conventional silicon carbide are quoted, for example, by Xu et al., IITC Conference Proceedings, pp. 109-11, (June, 1999) who disclose an alternative barrier/etch stop film composed of silicon, carbon and hydrogen. There remains, however, a need for a material with a low leakage current and high breakdown field that combines the hardness benefits of silicon nitride as an etch stop layer with a dielectric constant lower than that of silicon nitride.
An amorphous material containing silicon, carbon, hydrogen and nitrogen, denoted a-Si:C:H:N, provides a barrier/etch stop layer for use with low dielectric constant insulating layers and copper interconnects. The material has an elemental composition that is about 15 to 40% silicon, about 20 to 40% carbon, about 25-55% hydrogen, and about 2-20% nitrogen. The amorphous material is prepared by plasma assisted chemical vapor deposition (CVD) of alkylsilanes, such as tetramethylsilane, together with nitrogen and ammonia. Useful process conditions include flow rates of alkylsilane, nitrogen, and ammonia each in the range of about 500 to 2000 standard cubic centimeters per minute (sccm). The ratio of the sum of nitrogen and ammonia flow rates to the alkylsilane flow rate varies between about 0.25:1 and about 2.75:1. Favorable results have been obtained under conditions in which ammonia constitutes at least about 15% of the sum of the nitrogen and ammonia flow rates.
The deposition process provides an a-Si:C:H:N material that at the same time has a dielectric constant less than about 6 and favorable electrical and mechanical properties. Films with a dielectric constant less than about 4.5, that do not experience electrical breakdown at a field strength of about 5 MV/cm, and that have a leakage current less than or on the order of 1 nA/cm2 at a field strength of 1 MV/cm have been obtained. The films have excellent mechanical properties; hardness greater than 8 Gpa and compressive stress as low as 50 Mpa. They also serve as an effective diffusion barrier; no copper diffusion was observed after 26 hours through a 500 xc3x85 thick film held at 250xc2x0 C. at an applied field of 1 MV/cm. In addition, the a-Si:C:H:N film has an etch rate in a standard fluorocarbon dry etch that is at least 2 times slower and as much as 10-12 times slower than the etch rate of silicon dioxide.
The amorphous material meets the requirements for use as a barrier/etch stop layer in a standard damascene fabrication process. The process includes depositing an etch stop layer of a-Si:C:H:N material on a substrate where the substrate has regions of metal conductor and regions of insulating material, depositing a layer of insulating material on the etch stop layer, and etching patterns in the layer of insulating material where the etch stop layer etches at a slower rate than the layer of insulating material. The patterns are then lined and filled with a conductor. In some processes, the amorphous material is also used as an etch stop material within the layer of insulating material, separating a trench level and a via level.